Mitochondrial DNA (mtDNA) survey reveal strong separation between the Brazilian and Carribean provinces, and no significant population structure across the oceanic gap separating Brazil and the mid-Atlantic ridge. The genetic differences between the Brazilian and Carribean populations is matched by colour differences: in the North Atlantic, individuals show a distinguishing arrow, bluish posterior margin on the caudal and dorsal fins, whereas in the South Atlantic, individuals have a bright yellow margin (Rocha et al. 2002). Morphological and genetic data indicate that Acanthurus bahianus is now split into two species: Acanthurus tractus, which is distributed in the northwestern Atlantic and A. bahianus distributed from eastern Brazil to Ascension and St. Helena Islands (Bernal and Rocha 2011).

Justification:
Morphological and genetic data indicate that Acanthurus bahianus is now split into two species: Acanthurus tractus, which is distributed in the northwestern Atlantic and A. bahianus distributed from eastern Brazil to Ascension and St. Helena Islands (Bernal and Rocha 2011). A. bahianus is common and abundant in parts of its range. Although the northern portion of its range is subject to fishing pressure, there is no information at the present time indicating that this species is experiencing localized declines from fishing in its range of distribution. It is found in a number of marine protected areas in parts of its range. It is therefore listed as Least Concern. Additional research is needed to determine the effects of harvesting to this species and to what extent it is being utilized in its range of distribution.

Acanthurus bahianus is found in the South and Central Atlantic off the coast of Brazil, from Parcel Manuel Luiz to Santa Catarina, including Fernando de Noronha, Atol das Rocas, Trinidade, Ascension and St. Helen (Bernal and Rocha 2011).

Acanthurus bahianus was the dominant roving herbivorous/detritivore species in terms of density (0.11 ind m-2) recorded from Abrolhos Bank eastern Brazil (Francino-Filho et al. 2009). It is common along the northern and central coasts of Brazil, at all but the smallest of the Brazilian offshore islands, and at both oceanic islands (Ascension and St. Helena) in the south-central Atlantic (Randall 2002, Rocha et al. 2002). It is one of the commonest small, herbivorous/detritivore reef-fishes throughout its range (Robertson et al. 2005b). It is one of the most abundant reef species in the outlying areas of its range (Ascension and St. Helena) (J.H. Choat pers. comm. 2010).

Morphological and genetic data indicate that Acanthurus bahianus is now split into two species: Acanthurus tractus, which is distributed in the northwestern Atlantic and A. bahianus distributed from eastern Brazil to Ascension and St. Helena Islands (Bernal and Rocha 2011). However, we infer that life history and biological information from the former northern portion of this species' range (now considered to be A. tractus), is still pertinent to this species.

Acanthurus bahianus inhabits coral reefs and inshore rocky areas, generally where mixed with sandy substrata. It grazes on many species of benthic algae, occasionally on seagrass, it also feeds on the film of algae on the surface of sand undisturbed by surge. Contents of digestive tract include from 5 to 80% inorganic material (Randall 2002). It is often found together actively feeding with A. coeruleus (Lawson et al. 1999), this information now applies to A. tractus (Bernal and Rocha 2011). It is found singly (territorial to some extent) or in roaming feeding schools, and do not form territorial pairs or harems as do Indo-Pacific Acanthurids that show sexual dimorphism (Robertson 1985). Schooling occurs primarily in adults; small juveniles never participate in large, dense schools. The proportion of adults that were schooling increased from the back reef to the reef crest to the spurs and grooves zone (Lawson 1999).

The pelagic juveniles of this species settle in sheltered shallow habitats (Robertson 1988, 1992). The pelagic larval stage ranges from 42 to 68 days (Rocha et al. 2002), it then settles on reefs when 26.9 mm (Robertson 1992). This species is generally non-aggressive and highly mobile (Lawson et al. 1999).

A study by Lawson et al. (1999) showed that this species exhibits ontogenetic habitat shifts. Density of small juveniles was highest in the back reef, lower on the reef crest and lowest in the spurs and grooves zone, but schooling adults were most abundant in the spurs and grooves zones.

There are strongly contrasting patterns of habitat variation in demography at 2 sites studied by Robertson et al. (2005a): at Bermuda, fish settle inshore, grow to about asymptotic size and then, when 2 to 6 yr old, relocate permanently to outer reefs, where they can reach 32 yr. At Belize, fish settle and attain 10 yr on both inner and outer reefs, but grow faster and reach a ~50% greater asymptotic weight on inner reefs.

Growth

A. bahianus displays the highly characterized pattern of asymptotic growth. Terminal size was reached at around age 4, and most of the growth occurred within the first 10% of the lifespan, and approximately 85% of somatic size was attained within the first year. The mean maximum age fluctuated from 5 years in Jamaica to 26 years in Bermuda with a maximum longevity of 32 years in Bermuda (Mutz 2006).

Throughout this species' large latitudinal range growth occurs for a relatively short and fixed period (2 to 3 yr) and then effectively ceases, despite the existence of considerable variation in maximum longevity. It exhibits the fastest growth known for an Acanthurid, and the strongest spatial variation in demography known for tropical reef fish. Maximum age, adult survivorship, terminal size and absolute growth rate are inversely related to temperature (Robertson et al. 2005a).

A study by Choat & Robertson (2002) show the following maximum age estimates from different locations of this species' range:

A. bahianus does not exhibit sexual size dimorphism and has a 1:1 sex ratio (Reeson 1983). There are also no sexual difference in colour or shape in this species (Robertson 1985). Maturity seems likely to occur within 1 year in most cases (Robertson et al. 2005a). First maturity is at about 11 cm (FL), and most fish are probably mature at 15 to 16 cm (FL) (Reeson 1983). It was observed to form a resident spawning aggregation of up to 20,000 individuals each afternoon from December to March off southwestern Puerto Rico over 4 years (Colin 1985, Colin and Clavijo 1988). Fish spawned as subroups out of the larger aggregation in a manner identical to A. nigrofuscus. It also pair spawns, the males holding small territories adjacent to the aggregation site (Domeier and Colin 1997).

Morphological and genetic data indicate that Acanthurus bahianus is now split into two species: Acanthurus tractus, which is distributed in the northwestern Atlantic and A. bahianus distributed from eastern Brazil to Ascension and St. Helena Islands (Bernal and Rocha 2011). However, we infer that fisheries information from the former northern portion of this species' range (now considered to be A. tractus), may still apply to this species. Additional research is needed to determine the extent of the utilization of this species.Acanthurus bahianus is a component of subsistence fisheries and is caught in traps and gill nets (as by catch) and occasionally by spearing (Randall 2002). Trapping is the primary method by which this species is fished (Robertson et al. 2005b). This species is not a targeted catch and the method (traps) is not widely used in San Blas (Choat and Robertson 2002a). It is also a component of the aquarium trade (Global Marine Aquarium Database).

There is no information at present time indicating that this species is experiencing localized declines from fishing in parts of its range. Additional research is needed to determine the effects of harvesting to this species and to what extent it is being utilized in its range of distribution.

Surgeonfishes show varying degrees of habitat preference and utilization of coral reef habitats, with some species spending the majority of their life stages on coral reef while others primarily utilize seagrass beds, mangroves, algal beds, and /or rocky reefs. The majority of surgeonfishes are exclusively found on coral reef habitat, and of these, approximately 80% are experiencing a greater than 30% loss of coral reef area and degradation of coral reef habitat quality across their distributions. However, more research is needed to understand the long-term effects of coral reef habitat loss and degradation on these species' populations. Widespread coral reef loss and declining habitat conditions are particularly worrying for species that recruit into areas with live coral cover, especially as studies have shown that protection of pristine habitats facilitate the persistence of adult populations in species that have spatially separated adult and juvenile habitats (Comeros-Raynal et al. 2012).

In-Place Research, Monitoring and PlanningIn-Place Land/Water Protection and Management Conservation sites identified:Yes, over part of range Occur in at least one PA:YesIn-Place Species ManagementIn-Place Education

Lawson, G.L., Kramer, D.L. and Hunte, W. 1999. Size-related habitat use and schooling behavior in two species of surgeonfish (Acanthurus bahianus and A. coeruleus) on a fringing reef in Barbados, West Indies. Environmental Biology of Fishes 54: 19-33.